Frequency Tunable, Cavity-Enhanced Single Erbium Quantum Emitter in the Telecom Band

Yong Yu, Dorian Oser, Gaia Da Prato, Emanuele Urbinati, Javier Carrasco Ávila, Yu Zhang, Patrick Remy, Sara Marzban, Simon Gröblacher, and Wolfgang Tittel

Phys. Rev. Lett. 131, 170801 – Published 23 October 2023

Abstract

Single quantum emitters embedded in solid-state hosts are an ideal platform for realizing quantum information processors and quantum network nodes. Among the currently investigated candidates, ${Er}^{3 +}$ ions are particularly appealing due to their $1.5 μ m$ optical transition in the telecom band as well as their long spin coherence times. However, the long lifetimes of the excited state—generally in excess of 1 ms—along with the inhomogeneous broadening of the optical transition result in significant challenges. Photon emission rates are prohibitively small, and different emitters generally create photons with distinct spectra, thereby preventing multiphoton interference—a requirement for building large-scale, multinode quantum networks. Here we solve this challenge by demonstrating for the first time linear Stark tuning of the emission frequency of a single ${Er}^{3 +}$ ion. Our ions are embedded in a lithium niobate crystal and couple evanescently to a silicon nanophotonic crystal cavity that provides a strong increase of the measured decay rate. By applying an electric field along the crystal $c$ axis, we achieve a Stark tuning greater than the ion’s linewidth without changing the single-photon emission statistics of the ion. These results are a key step towards rare earth ion-based quantum networks.

Received 28 April 2023
Accepted 20 September 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.170801

Physics Subject Headings (PhySH)

Quantum information with atoms & light Quantum networks Stark effect

Nanotechnology Optical microcavities Rare-earth ions

Time-resolved photoluminescence

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Yong Yu ¹, Dorian Oser^2,*, Gaia Da Prato ¹, Emanuele Urbinati¹, Javier Carrasco Ávila ^3,4, Yu Zhang¹, Patrick Remy ⁵, Sara Marzban^2,†, Simon Gröblacher ^1,‡, and Wolfgang Tittel^2,3,4,§

¹Kavli Institute of Nanoscience, Department of Quantum Nanoscience, Delft University of Technology, 2628CJ Delft, The Netherlands
²QuTech, Delft University of Technology, 2628CJ Delft, The Netherlands
³Department of Applied Physics, University of Geneva, 1211 Geneva, Switzerland
⁴Constructor University Bremen, 28759 Bremen, Germany
⁵SIMH Consulting, Rue de Genève 18, 1225 Chêne-Bourg, Switzerland

^*Present address: QphoX B.V., 2628XG Delft, The Netherlands.
^†Present address: MESA+ Institute for Nanotechnology, University of Twente, 7500AE Enschede, The Netherlands.
^‡s.groeblacher@tudelft.nl
^§wolfgang.tittel@unige.ch

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Vol. 131, Iss. 17 — 27 October 2023

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Images

Figure 1
Experimental setup. (a) A scanning electron microscopic image of the PIC, consisting of two PhC cavities and a tapered waveguide in the middle as a common coupler. (b) Schematic of the device. The silicon PIC is placed on an Er:LN crystal and is optically accessed via a lensed fiber. A pair of electrodes above and below the LN crystal allows applying an electric field. The top right inset shows a simplified energy level scheme of Er:LN at no magnetic field. The crystal field splits the ${{}^{2 S + 1}L}_{J}$ states of the free ${Er}^{3 +}$ ion (green lines) into $J + 1 / 2$ Kramers doublets (black lines). The $Z_{1} - Y_{1}$ transition is at around 1532 nm. (c) Optical setup for measuring PL. Three concatenated acoustic optical modulators (AOMs) are used to create excitation pulses from a continuous wave laser. A $99 ∶ 1$ beam splitter (BS) routes the excitation pulse to the DUT and guides the light emitted from the sample to the detection setup. After an optical chopper, which blocks the reflected excitation pulses, a SNSPD detects the PL signal. (d) Time sequence of the experiment. The exponentially decaying PL (blue) is collected after a short excitation pulse (red) is sent.
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Figure 2
Characterization of ensemble ${Er}^{3 +}$ ions. (a) PL spectrum of the ${Er}^{3 +}$ ions under the coupler waveguide. The gray curve shows a Gaussian fit. (b) Time-resolved PL of ions with PhC cavity enhancement (blue, cavity resonance at 1532.5 nm) and without enhancement (red). (c) Simulated average cavity-addressable ion number $N$ within a frequency interval of 4 GHz as a function of the cavity resonance frequency. The star marks the frequency used in the single-ion measurements. The gray shaded area indicates the frequency region where spectrally resolved single ions are expected, with two blue dashed lines identifying $N = 1$ and 100.
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Figure 3
Characterization of ${Er}^{3 +}$ ions in the few- or single-ion regime. (a) PL spectrum of ions coupled to a PhC cavity. The spectrum is measured with 1 MHz precision and each data point is accumulated for 6 s. The inset shows an enlarged spectrum of the PL peak labeled ion 2 (blue dots) and a Gaussian fit (gray line) with 11.1 MHz FWHM. The units of the axes are as in the main figure. (b) Time-resolved photon emission from the cavity-enhanced PL line “ion 1” (blue). The PL decay of the ${Er}^{3 +}$ ensemble that couples only to the waveguide is shown for comparison (red). The inset shows the decay using another PhC cavity with a decay constant of $12.5 \pm 3.9 μ s$ [ $g^{(2)} (τ)$ not measured for noisy data [45] ]. (c) Second-order auto-correlation function $g^{(2)} (τ)$ measured during 30 min with photons from PL line “ion” 1 using a Hanbury Brown–Twiss setup. We group all photon detections during $90 μ s$ after each excitation pulse into a single time bin, and the horizontal axis shows the autocorrelation coefficient as a function of the difference between excitation pulses. The error bars represent one standard deviation. The gray shaded area indicates the single emitter regime and the black dashed line shows the dark count contribution.
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Figure 4
Linear Stark tuning of a single ${Er}^{3 +}$ ion. (a) The spectral line of a single ${Er}^{3 +}$ ion shifts back and forth when a triangular voltage ramp is applied across the Er:LN crystal. (b) Frequency shift and autocorrelation coefficient $g^{(2)} (0)$ of emitted photons as a function of the electric field amplitude. The red dashed line is a linear fit to the frequency shift of the ion; error bars are calculated from data taken during different ramps. The error of the correlation coefficient is calculated from Poissonian photon counting statistics.
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Figure 5
(a) A spectral hole of 130 kHz width “burned” with minimum power into the inhomogeneously-broadened absorption line of Er:LN at $T = 1.1 K$ and $B = 1 T$ (inset), and hole widths $Γ_{hole}$ as a function of burning power (main graph). For comparison, panel (b) shows a 3.2 MHz-wide hole created under the same conditions but at $B = 0 T$ . Extrapolating the data in the main graphs to zero burning power, we find power broadening-free hole widths of $122 \pm 4 kHz$ and $3.02 \pm 0.10 MHz$ . Error bars are smaller than each data point.
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